Breast cancer is a heterogeneous disease comprised of at least 5 major tumor subtypes that coalesce as the 2nd leading cause of cancer death in women in the United States. Amongst individual breast cancer subtypes, those classified as being triple-negative breast cancers (TNBCs) are clinically unique via their presentation of aggressive phenotypes, and their high propensity to recur rapidly following conventional chemotherapy treatment. TNBCs are also noteworthy by their failure to express hormone receptors (estrogen and progesterone) and ErbB2/HER2, a phenotype that renders targeted chemotherapies (e.g., hormonal or HER2-directed) ineffective and contributes to the poor prognosis of TNBC patients. Although our understanding of the molecular features and clinical manifestations of TNBCs has increased in recent years, science and medicine still lack sufficient knowledge of TNBC development and metastasis to permit the synthesis of chemotherapies capable of specifically targeting and alleviating this aggressive breast cancer subtype. WAVE3 is a member of the WASP/WAVE family of actin-binding proteins and plays an essential role in governing cell shape/morphology, actin polymerization and cytoskeleton remodeling, and cell motility and invasion. Importantly, we showed that WAVE3 expression is aberrantly elevated in TNBCs, and that enforced expression of WAVE3 promotes the acquisition of EMT, invasive, and metastatic phenotypes in this aggressive BC subtype. Based on these and other preliminary findings, we hypothesize that (i) c-Abl-mediated phosphorylation of WAVE3 promotes the aggressiveness of late-stage TNBCs; (ii) Interaction between WAVE3 and YBox1 (YB1), a cancer stem cell-specific transcription factor, enhances the development and progression of TNBC tumors; and (iii) cellular depletion of WAVE3 expression sensitizes TNBCs to the anticancer and apoptotic activities of standard-of-care chemotherapies. These hypotheses will be addressed by three Specific Aims. Aim 1 will determine the role of c-Abl-mediated phosphorylation of WAVE3 on TNBC development and progression. We will create c-Abl-resistant WAVE3 mutants and determine their ability to regulate TNBC tumorigenicity both in vitro and in vivo. Mass-spectrometry analyses have identified novel WAVE3-binding proteins, such as YB1. Thus, Aim 2 will investigate the significance of the WAVE3/YB1 interaction in promoting the development and metastatic progression of TNBCs, and in the regulation of the transcription machinery of cancer stem cells, which will be determined in in vitro and in vivo models of TNBCs. Aim 3 will use our newly developed ECO nanoparticles to specifically deliver WAVE3 inhibitors to TNBC tumors and determine the effectiveness of WAVE3 inactivation to restore/enhance the sensitivity of TNBCs to chemotherapy as a means to alleviate TNBC progression and metastasis. Collectively, the findings obtained in this innovative application will provide novel molecular insights into how WAVE3 promotes TNBC tumorigenicity. They will also generate innovative translational outcomes in the form of novel treatment options using WAVE3 inhibitors to resensitize TNBCs to standard-of-care chemotherapies and to alleviate TNBC development and metastatic progression.